Mammalian torpor and depressive disorders: a review of evidence for a connection

Unexpected similarities exist between animal torpor and depressive disorders. We examine them from a variety of perspectives and propose that these two seemingly disparate entities are related in important ways. In the animal kingdom, torpor - a hypometabolic state during which physiological functions are severely decreased - is now understood to be a response to stress, whether unanticipated or seasonal. It is used by mammals from the tropics to the poles, including by several species of primates. Humankind evolved from mammals that used torpor, and the genetic legacy enabling the torpor response exists in every person's genome. Strikingly, severe cold has occasionally created a torpor-like state in human beings - and 'synthetic hibernation' is now being studied as a modality to facilitate space travel. The expression of torpor can vary wildly among species and even between any two individuals within the same species living in the same locale, implying unusual mutability of this complex and variegated adaptation and the likely expression of aspects of it in vulnerable members of our own species. Both torpors and depressions require perturbation of the circadian clock, which is resynchronized upon recovery. The same types of prominent but reversible hippocampal changes mark both depressions and torpors. The circannual clock that drives seasonal torpors also appears to guide some recurrent affective disorders. Mammals can be brought out of their torpors by the same interventions that bring people out of their depressions. Affective catatonias and certain subtypes of depressions display aspects of a torpor-like response and are explored. Torpor arguably is the only animal model of depression that fulfills Willner's triad of face, construct, and predictive validity (plus etiological similarity, a criterion added later). Some aspects of depressive disorders, puzzling at first, can be explained if one views them from the perspective of a torpor response. Ideas for future research that might elucidate the connections between depressive disorders and torpor are suggested. The proposed connection between torpor and certain subtypes of depressive disorders warrants greater attention by researchers in these currently disconnected fields. A collaborative approach could lead to better treatments for depressed persons.